Resin compositions for optical-semiconductor-element encapsulation, which are used for encapsulating optical semiconductor elements such as light-emitting diodes (LEDs), are required to give a cured resin having transparency. In general, epoxy resin compositions obtained from an epoxy resin, such as a bisphenol A epoxy resin or an alicyclic epoxy resin, and an acid anhydride hardener have been commonly used (see, for example, JP-A-2006-274249).
However, there are cases where the epoxy resins discolor when used over long at high temperatures, resulting in a decrease in luminance of the light-emitting diode devices. Encapsulating resins are required to have a modulus of elasticity reduced to some degree in order to stably produce the resins in a sheet form by a roll-to-roll process or to relieve the stress to be imposed on the optical semiconductor devices obtained by encapsulation with the resins. However, epoxy resins generally have a high modulus of elasticity and are hence unsuitable for that purpose.